1. Field of the Invention
This invention relates to integrated circuit manufacture and more particularly to multi-layered contact structures using germanide as a way in which to lower resistivity and control growth of silicide over an ultra-shallow junction.
2. Background of the Relative Art
An integrated circuit is by definition a number of electrically interconnected circuit elements defined on the same substrate or "chip". Some of the interconnections are done in the silicon substrate itself, but most are done my means of thin conductive strips running across the top surface of the substrate. Each strip is often connected within a contact area to underlying semiconductor materials (often referred to as "junctions"). Contact to junctions must be of low resistivity and is generally as low as a few micro ohms per square centimeter of contact area.
The conductive strips are usually made of aluminum or aluminum alloy, and, in some instances, can have silicon placed therein. Aluminum adheres well to silicon dioxide and has low contact resistance, but may suffer numerous problems, such as, for example, a propensity to grow "spikes".
In ultra-shallow regions (i.e., junctions having a thickness less than, for example, 1,000 angstroms), aluminum may spike completely through the underlying junction at the contact area. To prevent spiking, the contact structure must be altered with, for example, a sacrificial, passive or stuffed barrier material. The barrier material resides between the aluminum and underlying silicon. A popular barrier comprises titanium nitride, wherein the nitride stuffs the grain boundaries of the titanium thereby preventing a substantial amount of silicon diffusion into the overlying aluminum from the junction region. As the junction region becomes shallower in accordance with modern day technologies, it is important the barrier remain configured between the underlying silicon and overlying aluminum. However, the barrier must be formed in a low-temperature ambient so as to not further deepen the shallow junction region.
Along with barrier materials, the contact structures further include steps for lowering the contact resistance at the interconnect/silicon juncture. Specifically, most manufactures utilize a silicide formed at the juncture. The silicide helps break through the residual surface oxide so that good electrical contact can be made. Applying heat necessary for silicidation is sometimes required to adjust the silicon dioxide--silicon interface states. Silicides are made by depositing a thin layer of metal over the entire wafer, heating the wafer to a high enough temperature for the silicon and metal to react in the contact window areas and then etching away the unreacted metal on top of the oxide. Most metals used to form silicide are transition or refractory metals in group IV(B), V(B) and VI(B).
By depositing a refractory metal, such as titanium, across the wafer and then heating the titanium, it reacts with underlying silicon to form a silicide. Simultaneously, nitrogen atoms are inserted into the upper surface of the titanium to stuff the grain boundaries and provide barrier functionality. For this reason, titanium provides suitable properties for both silicide and barrier formation in a single anneal step. Unfortunately, the advent of ultra-shallow junctions has lead to many constraints on the silicidation of titanium. First, conventional processes require an anneal greater than approximately 800.degree. C. to form titanium silicide (TiSi.sub.2). Any temperature less than 800.degree. C. can produce non-stoichiometric silicide leading to greater sheet resistance in the contact area. However, at more suitable silicide temperatures approaching 800.degree. C., highly mobile boron dopants within the junction diffuse at greater diffusion lengths causing a deepening of the junction. Deepening of the junction can result in greater parasitic source/drain capacitance and lower breakdown voltages. Additionally, driving away of boron impurities from the surface area to deeper positions can increase the contact and sheet resistance in the contact area. Still further, increased heat can cause boron to diffuse along with silicon atoms into the growing silicide and further deplete boron at the silicon surface (adjacent the lower edge of silicide). Boron depletion caused by high temperature processing or by silicide-induced consumption must be minimized.
It is also important that the silicide be grown to a controlled thickness. If the silicide film becomes too thick, defects can occur at the edge of the silicide film due to stresses in the film. Such defects are reported to begin occurring once the thickness of the silicide film exceeds approximately 100 nm. The mechanism for growing silicide is generally understood as species of silicon diffusing from the underlying substrate surface to the overlying (and abutting) titanium. If an excessive amount of silicon atoms are allowed to diffuse, then the silicide is made too thick causing undo stresses in the film. It is therefore important not only to minimize the silicide growth temperature, but also to prevent excessive silicon consumption during the growth process. It is important that the integrity of the boron atoms, once placed, remain in their position to maintain the ultra-shallow junction region and the advantages thereof.
The above problems generally present themselves whenever titanium is used as the base silicide metal, and the underlying junction is p-type, highly mobile boron atoms. While use of titanium presents many problems, there remains many advantages of titanium. Titanium is a mainstay and essential component in modern contact structures. Nitrogen-doped titanium has high quality stuffed barrier properties. Titanium juncture to underlying silicon forms low resistance silicide--an important feature of all contact structures. While it would be advantageous to continue utilizing titanium silicide and the nitrogen-stuffed titanium barrier, it would be further advantageous to utilize such materials with modern day ultra-shallow junctions. Specifically with junctions that have highly mobile boron atoms therein.